Kofron et al U.S. Pat. No. 4,439,520 was the first to demonstrate that tabular grain emulsions are capable of providing a variety of photographic advantages, including improvements in photographic sensitivity and speed-granularity relationships.
Solberg et al U.S. Pat. No. 4,433,048 was the first to demonstrate that tabular grain emulsions with higher iodide concentrations adjacent the peripheral edges of the tabular grains are capable of demonstrating photographic sensitivities higher than those of comparable tabular grain emulsions containing the same overall iodide concentrations, but uniformly distributed. Subsequently others have investigated tabular grain emulsions with non-uniform iodide distributions in which the highest iodide level occurs at a surface location, as illustrated by the following: Hayakawa U.S. Pat. No. 4,883,748, Piggin et al U.S. Pat. Nos. 5,061,609 and 5,061,616, Bell et al U.S. Pat. No. 5,132,203, Bando U.S. Pat. No. 5,206,133 and Brust et al U.S. Pat. No. 5,314,798.
Corben U.S. Pat. No. 4,210,450 discloses the preparation of a shelled converted halide emulsion by alternately ammoniacally precipitating silver chloroiodobromide and introducing ammonium iodide and then repeating the sequence. The emulsions are stated to be useful in color diffusion transfer, but no performance advantages are stated or demonstrated.
Chaffee et al U.S. Pat. No. 5,358,840, discloses a tabular grain emulsion in which iodide is present adjacent central portions of the tabular grain major faces extending to a depth of 0.02 .mu.m in a concentration in excess of 6 mole percent with overall iodide concentration of the tabular grains being in the range of from 2 to &lt;10 mole percent.